Acrocentric: See chromosome.
Activating mutation: See mutation.
Activator: A protein that functions to improve gene transcription.
Affordable Care Act: Complex Federal legislation aimed at ensuring that all Americans have practical access to effective health care coverage.
Agenesis: Referring to embryonic formation. Agenesis is the failure of a specific structure to develop. (e.g., agenesis of the corpus callosum means the corpus callosum did not develop at all.)
Hypogenesis: The underdevelopment of a specific structure. (e.g., hypogenesis of the corpus callosum refers to a smaller than normal corpus callosum.)
Allele(s): Alternative forms of a gene, or of a DNA sequence, at a given locus.
Null allele: When applied to a gene that codes for an enzyme, the null allele changes the gene to produce zero enzyme product.
Allelic heterogeneity: See heterogeneity.
Allosteric control: The regulation of an enzyme or other metabolic function via an effector molecule at a site other than the protein's active site (e.g., an allosteric site).
Allosteric protein: A protein that alters its shape upon binding with another molecule.
Alpha-satellite: Repeating units of DNA which are about 170 bp. They are found mainly in the heterochromatic region around the centromeres.
Alternative splicing: The process of excising introns in different patterns to produce alternate DNA forms for transcription and ultimately resulting in different proteins from the same core sequence.
Anaphase: See cell cycle.
Aneuploid: Not possessing the correct number of chromosomes. There are several descriptors of such abnormalities (also see "Ploidy"). Examples would include:
Disomy: Having two copies of one chromosome.
Monosomy: Having one copy of one chromosome.
Trisomy: Having three copies of one chromosome.
Anneal: The coupling of complementary strands of DNA.
Re-anneal: The coupling of complementary DNA strands after having first been separated.
Anticipation: The apparent worsening of a heritable condition as it is passed down through generations.
Apoptosis: Programmed cell death (sometimes referred to as "cellular suicide").
Array comparative genomic hybridization (aCGH): See comparative genomic hybridization.
Association: Two or more malformations, which have been found (on a population basis) to occur together more often than would be predicted by chance, but not necessarily due to a specific causal entity.
Autosome (autosomal): Referring to one of the 22 numbered (non-sex) chromosomes.
Auxology (auxologic): The study of human growth.
Auxotrophy: The inability to synthesize all of the biochemical necessary for all of the functions in an organism. (An auxotroph is an organism that exhibits auxotrophy.)